A radiotherapy device generally includes a linear electron beam accelerator which is mounted on a gantry and which can rotate about an axis which is generally parallel to the patient lying on the patient couch. During the radiation therapy, the patient is treated using either an electron beam or an X-Ray beam produced from the original electron beam. The electron or X-Ray beam is focused at a target volume in the patient by the combination of the use of a collimator and the rotation of the beam. The patient is placed on a couch which can be positioned such that the target lesion can be located in the plane of the electron beam as the gantry rotates in two directions.
The objective of the radiation therapy is to target the lesion with a high dose of radiation over time and to have minimal impact on all the surrounding normal tissue. The first task is to precisely locate the tumor in three dimensional space. The best technique for this is MRI since this technology provides high resolution in the imaging of soft tissue to provide high soft tissue contrast.
Even though MRI provides good location of the tumor at the time of the measurement, these images are normally recorded two to three days prior to the treatment and so may not be completely representative of tumor location on the day of treatment. This is because the movement of the patient over time can cause the anatomical location of the tumor to move. The oncologists therefore tend to increase the target volume to be certain that all of the tumor tissue receives the required dose of the radiation, even though this increase in the volume of the tissue exposed to radiation also necessarily targets healthy tissue with consequential damage to the healthy tissue. The expectation is that all cells in the targeted region will be killed and this includes both the lesion and the healthy tissue. This produces collateral damage and may have a significant impact of the quality of life of the patient.
An additional challenge to effective radiation treatment is the effect of motion of the tumor in the body due to respiratory and cardiac motion. This results in tumor masses moving making the continuous accurate targeting for treatment difficult. Again therefore the oncologists generally increase the size of the target volume radiated to accommodate movement of the lesion during respiratory and cardiac movement.
A number of attempts have been made to improve the accuracy of the location of the lesion for radiotherapy.
U.S. Pat. No. 5,178,146 (Giese) issued Jan. 12, 1993 discloses a grid system of contrast material which is compatible with MRI which is used to plan radiotherapy.
The following patents disclose a technique for identifying the target volume using MRI which is used to plan radiotherapy;
U.S. Pat. No. 5,402,783 (Friedman) assigned to Eco-Safe and issued Apr. 4, 1995;
U.S. Pat. No. 5,537,452 (Shepherd) issued Jul. 16, 1996;
U.S. Pat. No. 5,800,353 (McLaurin) issued Sep. 1, 1998;
U.S. Pat. No. 6,198,957 (Green) assigned to Varian and issued Mar. 6, 2001;
A number of attempts have been made to compensate for the movement of the lesion during the irradiation.
U.S. Pat. No. 6,725,078 (Bucholz) assigned to St Louis University and issued Mar. 6, 2001 discloses a combined MRI and radiotherapy system which operate simultaneously but without interference so that the location of the lesion can be tracked during the radiotherapy.
U.S. Pat. No. 6,731,970 (Schlossbanner) assigned to BrainLab and issued May 4, 2004 discloses a method for breath compensation in radiation therapy, where the movement of the target volume inside the patient is detected and tracked in real time during radiation by a movement detector. The tracking is done using implanted markers and ultrasound.
U.S. Pat. No. 6,898,456 (Erbel) assigned to BrainLab and issued May 24, 2005 discloses method for determining the filling of a lung, wherein the movement of an anatomical structure which moves with breathing, or one or more points on the moving anatomical structure whose movement trajectory is highly correlated with lung filling, is detected with respect to the location of at least one anatomical structure which is not spatially affected by breathing, and wherein each distance between the structures is assigned a particular lung filling value. There is also disclosed a method for assisting in radiotherapy during movement of the radiation target due to breathing, wherein the association of lung filling values with the distance of the moving structure which is identifiable in an x-ray image and the structure which is not spatially affected by breathing is determined, the current position of the radiation target is detected on the basis of the lung filling value, and wherein radiation exposure is carried out, assisted by the known current position of the radiation target.
U.S. Pat. No. 7,265,356 (Pelizzari) assigned to University of Chicago and issued Sep. 4, 2007 discloses an image-guided radiotherapy apparatus and method in which a radiotherapy radiation source and a gamma ray photon imaging device are positioned with respect to a patient area so that a patient can be treated by a beam emitted from the radiotherapy apparatus and can have images taken by the gamma ray photon imaging device. Radiotherapy treatment and imaging can be performed substantially simultaneously and/or can be performed without moving the patient in some embodiments.
U.S. Pat. No. 7,356,112 (Brown) assigned to Elektra and issued April 8, 2008 discloses that artifacts in the reconstructed volume data of cone beam CT systems can be removed by the application of respiration correlation techniques to the acquired projection images. To achieve this, the phase of the patients breathing is monitored while acquiring projection images continuously. On completion of the acquisition, projection images that have comparable breathing phases can be selected from the complete set, and these are used to reconstruct the volume data using similar techniques to those of conventional CT. This feature in the projection images can be used to control delivery of therapeutic radiation dependent on the patient's breathing cycle, to ensure that the tumor is in the correct position when the radiation is delivered.
The same company Elektra AB of Stockholm Sweden, as set out in an undated page taken from their web site, have developed a machine using CT guided radiation where CT is used to image the patient just prior to irradiation. They state that better margins can be set using Motion View sequential imaging.
There are previous proposals for using MRI magnets to monitor treatment using electron beams created by a linear accelerator. The problem with this is the non-compatibility of linear accelerators and MRI. This arises because the magnetic field generated by the magnet of course interferes with the operation of the linear accelerator to an extent which cannot be readily overcome. It has however been found that relatively low field MRI units can be used with gamma radiation produced from cobalt −60.
In U.S. Pat. No. 5,735,278 (Houllt et al) issued Apr. 7, 1998, is disclosed a medical procedure where a magnet is movable relative to a patient and relative to other components of the system. The moving magnet system allows intra-operative MRI imaging to occur more easily in neurosurgery patients, and has additional applications for liver, breast, spine and cardiac surgery patients.